In packet-switched networks, the packets serving the information transfer utilize the same communication resources, which can transmit a single packet at a time. As a result, the packets must queue for their allocated transmission position in the packet transmission sequence, which causes transmission delay.
In a multi-hop packet network a packet typically travels from the source node to the destination node via one or more other nodes, since the source and destination nodes are normally not adjacent nodes in the network and therefore direct communication between said nodes is not possible. Since every additional hop (i.e. transmission path between adjacent nodes) generates a transmission delay equal to at least one frame, the delay accumulates considerably in a multi-hop network.
In a synchronized packet network the nodes agree on how time is divided into “slots” and how the slots are grouped into “frames”. FIG. 1 illustrates these basic concepts of a synchronized network. In a typical synchronized network, the nodes share a common time sequence, such as a frame, that repeats regularly in the time domain. A single frame contains a predetermined number of time slots, which are divided between control (i.e. signaling) information and user data, the division being typically such that a certain predefined number of time slots, such as 10% of a frame time, is devoted to control packet transmission and the rest to data packet transmission. The length of a packet as compared to a time slot may vary in various systems.
A synchronized multi-hop packet network of the above kind is depicted for example in the PCT application WO 00/48367.
In this type of a network, a node and its neighboring nodes form a neighborhood. Thus, there are as many neighborhoods as there are nodes, although the neighborhoods overlap to a great extent. In the control portion of a frame a node agrees about the time slot reservations with all the nodes within its neighborhood. Each time slot in the control portion of a frame is typically allocated to a single node. In this time slot the node sends control packets including scheduling information, i.e. this information indicates how the node is scheduled to transmit and/or receive during the subsequent frames. Thus, in the control part scheduling for data packets in the current frame and in the consecutive frames is agreed upon among the nodes in the neighborhood.
When a packet starts its travel over the network, it will normally be scheduled to the first available free time slot in the neighborhood of the transmitting node. When the receiving node receives the packet, it will do the corresponding reservation in its own neighborhood. Since these reservations involve negotiations between the nodes, it is apparent that normal reservations on a packet-by-packet basis are not sufficient to meet the delay requirements for real-time and interactive services.
In order to support these services, persistent reservations have been introduced into the network. Persistent reservations are time slot reservations made for a longer period, and these persistent time slots are used to accommodate delay sensitive traffic, such as traffic generated by real-time or interactive services. Thus, persistent flows are chosen to convey delay sensitive traffic. Persistent flows are used in the above-mentioned PCT application, for example.
However, a drawback relating to the present networks is that they do not provide a controlled way for minimizing delay or delay variation, but the performance of the system is more or less coincidental in view of delay and delay variation. Thus, the persistent time slots cannot guarantee a good performance in terms of transmission delay.
A conventional method for decreasing the delay in multi-hop networks is to reduce the frame size in order to shorten the buffering time in a node. However, there is a minimum for the frame length, as signalling overhead becomes dominant when the frame length is reduced. In other words, the proportion of signalling information of the total transmission capacity becomes too high if the frame length is not above a certain minimum value. Further, the implementation constraints, such as the processing power available in a node, will simply define the minimum frame size that can be processed in real-time. Thus, the shorter the frames are, the higher is the processing power requirement in individual nodes. This drawback also relates to the number of time slots in a frame; if a certain number of time slots is required in the frame, the minimum length of a time slot which can still be processed sets a limit below which the frame length cannot go.
The objective of the present invention is to obtain a solution by means of which the above-mentioned drawbacks relating to short frames can be eliminated, and to bring about a scheme, which enables a simple and controlled way for minimizing delay and delay variation in multi-hop packet networks.